Despite considerable progress in early diagnosis and treatment of breast cancer, this disease remains a most significant cause of morbidity and mortality. A variety of oncogenes induce breast tumor formation when over expressed in the breast of transgenic mice including p21 ras, pp60src, c-MyC and activating mutations of ErbB2 (Neu). Understanding the molecular mechanisms governing cellular proliferation in breast cancer may provide important insights into alternate strategies for treatment of breast cancer. The cyclin D1 gene encodes a regulatory subunit of a holoenzyme (Cyclin D1-kinase,CD1 K) capable of phosphorylating and inactivating the tumor suppressor retinoblastoma protein (pRB). Cyclin D1 levels are increased in 70-100% of breast tumor cell lines and the majority of human breast cancers, and overexpression of cyclin D1 in the breast of transgenic mice induces breast tumor formation. Hormones known to be risk factors for human breast cancer, estrogen and progesterone, stimulate cyclin D1 levels. Recent studies by our laboratory and others have shown that several oncogenes capable of inducing breast cancer formation induce cyclin D1 expression. Cyclin D1 abundance is rate-limiting in the early proliferative step induced by serum in fibroblasts. It is currently unknown whether cyclin D1 is required for growth factor and oncogene induced cellular proliferation and tumor formation in breast cells. The goal of these studies is to understand the role of cyclin D1 in breast tumor formation. Cyclin D1 expression and kinase activity will be assessed in breast tumors derived from transgenic mice with oncogene induced breast tumors. In order to determine the requirement for cyclin D1 in breast tumor formation, transgenic mice will be established. In these animals, anti-sense cyclin DI expression will be controlled by a breast-specific (alpha lactalbumin), ligand-dependent (ecdysone) chimeric enhancer (ecdysone receptor (EcR]), retinoic acid X receptor, (RXR). These mice will be systematically back-crossed with mice transgenic for breast oncogenes (ErbB2 (Neu), pp60src, p21 c- Myc). These studies aim to determine whether a reduction in cyclin D1 levels is sufficient to reduce the prevalence of breast tumor formation in the offspring of the back-crossed mice. Establishing a breast specific inducible expression system in transgenic mice will provide a powerful and stringent bioassay for the efficacy of other tumor suppressor molecules in blocking breast tumor formation in subsequent experiments. Establishing whether cyclin D1 encodes a rate-limiting step in breast oncogenesis may pave the way for alternate molecular strategies for breast tumor therapy.